1. Field of the Invention
The invention relates to an optical scanning apparatus like a laser scanner unit carried on an image forming apparatus such as a copying machine or a printer, and particularly to an optical scanning apparatus having a scanning line curvature correcting mechanism capable of adjusting the amount of curvature of a mirror and correcting the curvature of a scanning line.
2. Related Background Art
As a color copying machine or a color printer using an electrophotographic recording technique, there is an apparatus in which a plurality of image forming portions are disposed in a tandem type. This tandem type image forming apparatus has the merit that it has relatively many available recording media and is high in recording speed, and is becoming the leading form of color image forming apparatuses in recent years.
Each of the image forming portions (usually four image forming portions) has a photosensitive member, a charger for charging the photosensitive member, an optical scanning apparatus for scanning the charged photosensitive member by a laser beam according to image information, a developing device for developing an electrostatic latent image formed on the photosensitive member with a toner, and a transfer charger for transferring a toner image formed on the photosensitive member to recording paper (or an intermediate transfer member).
In such a tandem type image forming apparatus, in order to suppress the color misregister of toner images of four colors superposed onto the recording paper (or the intermediate transfer member), it is necessary to correct the shape of a scanning line by the laser beam emerging from each optical scanning apparatus, and suppress the deviation of the shapes of the scanning lines. There are various correction items, and one of them is the correction of the curvature of the scanning line.
As the causes of the scanning line by the laser beam being curved, there are various causes such as the mounting tolerance of an fθ lens or a mirror carried on the optical scanning apparatus with respect to an optical box, and the manufacturing tolerances of these optical elements themselves. For example, on the optical scanning apparatus, there is carried a mirror for reflecting the laser beam deflected by a polygon mirror, but it is very difficult to manufacture the mirror so that there may be no warp in the glass substrate of this mirror, and usually there is some warp. This warp of the mirror is one of the causes of the scanning line being curved.
As a method of the correction of the curvature of the scanning line, there is a method of adjusting a degree of curvature of a laser beam reflecting mirror provided in the optical scanning apparatus. For example, Japanese Patent Application Laid-open No. H8-146325, Japanese Patent Applications Laid-open No. H10-186257, Japanese Patent Application Laid-open No. H11-231240, Japanese Patent Application Laid-open No. 2000-180778, Japanese Patent Application Laid-open No. 2000-235290, Japanese Patent Application Laid-open No. 2000-258713, Japanese Patent-Application Laid-open No. 2001-117040 and Japanese Patent Application Laid-open No. 2003-270573 disclose optical scanning apparatuses provided with mechanisms for adjusting the degree of curvature of a mirror (scanning line curvature correcting mechanisms).
Any of the curvature correcting mechanisms described in these patents adjusts the degree of curvature of the mirror by mirror pressure amount adjusting means provided in the longitudinally central portions or the longitudinally opposite end portions of the mirror to thereby correct the curvature of the scanning line.
For example, the optical scanning apparatus described in Japanese Patent Application Laid-open No. 2001-117040 is provided with pressure amount adjusting screws in the longitudinally opposite end portions of the mirror, and these screws are turned to thereby adjust the degree of curvature of the mirror.
Also, the optical scanning apparatus described in Japanese Patent Application Laid-open No. 2000-180778 is provided with a pressure amount adjusting screw in the longitudinally central portion of the mirror, and this screw is turned to thereby adjust the degree of curvature of the mirror.